1. Field of the Invention
The invention relates to a method for decoding F2F signals that are read from a magnetic data carrier, preferably a card carrying magnetic stripe (magnetic card), with the help of a magnetic read head. Furthermore, the invention relates to a device for decoding signals coded in F2F manner contained on a magnetic data carrier.
2. Description of Related Art
From U.S. Pat. No. 4,626,670 a method and system for decoding time-varying two-frequency (F2F), coherent phase-data, such as data from the stripe on a magnetic stripe-bearing medium is known which includes detecting flux intervals at the beginning of a bit cell of such data. This method further includes the timing of the length of such a bit cell with a clock or counter, digitally determining the average length of at least the two bit cells immediately preceding the cell being measured and digitally determining whether or not another flux reversal lies at or near the mid-point of the bit cell being decoded. Besides the fact that this known method differs structurally from the method used in the present invention, it seems also to be very sensitive to misinterpreting amplified noise or signal distortions that indicate cell boundaries where no cell boundaries are provided.
From U.S. Pat. No. 3,959,626 and U.S. Pat. No. 3,947,662 a method and apparatus is known for reading F2F coded data. The method and apparatus include means for simultaneously or separately accommodating both variable velocity scan or distorted reception conditions in the coded data or other variations in the spacing of transition signals in the recorded indicia. Frequency variations in transmitted data are accommodated. Correct interpretation of frequency-distorted transmitted data is made possible by optimizing the re-scanning time-point either out of F1 or F2. This known method and apparatus follows a different solution than the one presented by the present invention.
In the European patent application EP-A- 0 441 280 (U.S. Pat. No. 5,168,275, filed in the U.S. on Feb. 7, 1990) a method and apparatus for decoding two-frequency (F2F) data signals is described. There a two-frequency data signal, also known as a bi-phase or F2F signal, is accurately decoded by sampling the signal and digitizing the samples to provide a series of digital values representing the signal. An intelligent digital filter manipulates the digital values to decode the signal, by detecting the peak in the sample of the signal and decoding the signal by analyzing the location and amplitudes of the peaks. Only peaks which are outside a guard-band may be detected. If the signal cannot be properly decoded with a wide guard-band, the guard-band may be repeatedly narrowed, until a minimum guard-band is reached. Bits are identified by comparing the displacements between peaks to a bit cell width. An even number of displacements indicates a "0" bit, and an odd number of displacements indicates a "1" bit. Once a bit is decoded, the bit cell width is incremented by a predetermined amount if the decoded bit cell width is narrower than the current bit cell width. If no peaks are found within a maximum allowable bit cell width, this area is delimited as bad. An attempt is made to decode the bits in the bad area by analyzing all peaks without regard to a guard-band. After decoding, the bits are converted into bytes. Parity and longitudinal redundancy code checks are used to correct bad bits. During decoding, many indications of a degraded signal may be obtained. If a degraded signal is indicated, the host computer is notified, even if the signal was properly read. Card replacement can then be initiated at the first signs of signal degradation before the data signal becomes unreadable. The present invention follows a different method in signal decoding.
The signal quality of F2F encoded data is often bad. For example, frequent usage of a magnetic stripe card in financial applications(e.g., cash feeding at cash issuing terminals, payments at point of sales terminals, payment at gasoline stations, or use in a telephone cell for paying the bill), impacts very much on the signal quality of the magnetic stripe card. Therefore, amplitude fade and jitter may generate retries to read the card which further degrades the magnetic stripe. Sometimes the data on the card cannot be read any more. Different effects are responsible for the reduced quality of the read signal: fading, jitter, signal drop-outs or so-called glitches which represent additional spikes in the digitized signal.